Stents of both the balloon expandable and the self-expanding type are known that have been cut from metal cannula and expanded for placement, for example, in the vessels of a patient. In a number of designs, the stent can be comprised of first and second segments, one of which exhibits radial strength greater than that of the other. The lesser radial strength segment is then formed to have lateral flexibility greater than that of the first segment at least in the smaller diameter, unexpanded state or condition for delivery through tortuous vessels. This combination of segments provides a stent having both good radial strength as well as lateral flexibility.
One known stent is disclosed in U.S. Pat. No. 6,231,598 B1 issued May 15, 2001, and assigned to one of the assignees hereof. The stent is fabricated from cannula and is defined by one or more longitudinal segments of laterally interconnected closed cells. Each closed cell is defined laterally by a pair of longitudinal struts that are interconnected at each end by a circumferentially adjustable member that deforms to permit circumferential expansion while the length of the cell is maintained by the longitudinal struts. Adjacent ones of the longitudinal segments are joined by flexible interconnection segments that permit the stent to bend laterally, particularly in the unexpanded state, and that are comprised of curvilinear struts that form a series of serpentine bends that distribute lateral bending forces while only allowing a slight overall shortening of the stent. A short strut interconnects a longitudinal segment and an adjacent interconnection segment.
Other cannula stents are known from U.S. Pat. Nos. 5,421,955; 5,102,417; 5,928,280 and 5,195,984. A wire frame stent having a number of stent segments is disclosed in U.S. Pat. No. 5,104,404.
However, a problem associated with certain multiple segment stents is that relatively high tensile strains are produced therein that cause areas of metal fatigue. As a result, after these stents in the expanded state have been subjected to pulsatile expansion and contraction due to blood flow, the high tensile strain areas will eventually fracture. In addition, bending and torsional loads to which the stent is subjected when the patient changes physical position, can also cause metal fatigue and subsequent fracture. By way of example, these multiple segment stents have various peripheral vessel applications such as in the carotid of the patient. In addition, these peripheral stents can be subjected to external forces such as the patient having external pressure applied to a vessel and causing its collapse or deformation.
A further problem associated with certain multiple segment stents is that relatively high tensile strains are produced therein during radial expansion of the stent in manufacture. In particular, nitinol cannula tubes are laser cut to form the basic configuration of the stent in an unexpanded condition. The laser cut cannula stent is then radially expanded to a much larger diameter and then heat set to assume the shape of the larger diameter in a relaxed condition. During the radial expansion of the laser cut stent, significant tensile strain is experienced at various bends or strut interconnections of the stent. Depending on the design of the stent and, in particular, the sharpness of the bend angle, fractures, cracks, or gaps can readily occur in the cannula during the radial expansion of the stent. Significant analysis is typically done on the stent design to address pulsatile metal fatigue during the life of the stent in a patient. However, fractures, cracks, or gaps caused during the radial expansion of the stent in manufacture can result in significantly low manufacturing yields and increased costs of production. Furthermore, high levels of concentrated strain anywhere in the stent can readily lead to subsequent fracture during pulsatile contraction and expansion.
It is also a problem with cut cannula stents that the width of longitudinal portions and, in particular, longitudinal struts have a width that is wider or greater at the outside surface of the cannula tube than the inside passageway or lumen surface of the cannula tube. As a result, the cross-sectional area of the longitudinal strut is asymmetrical and fractures, cracks, and the like more readily form or occur at an inside surface edge of a cut cannula stent.